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Encyclopedia > Dalton (unit)

The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic masses and molecular masses. It is defined to be 1/12 of the mass of one atom of carbon-12. Accordingly, The word unit means any of several things: Physical unit, a fundamental quantity of measurement in science or engineering Units (computer program), a popular program that does unit conversion. ... Mass is a property of physical objects that, roughly speaking, measures the amount of matter they contain. ... ... The molecular mass of a substance (less accurately called molecular weight and abbreviated as MW) is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12). ... Properties For alternative meanings see atom (disambiguation). ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ...

1 u = 1/NA gram = 1/(1000 NA) kg   (where NA is Avogadro's number)
1 u ≈ 1.66053886 x 10-27 kg

See 1 E-27 kg for a list of objects which have a mass of about 1 u. The gram or gramme, symbol g, is a unit of mass, and is defined in the SI system of units as one one-thousandth of a kilogram (i. ... The international prototype, made of platinum-iridium, which is kept at the BIPM under conditions specified by the 1st CGPM in 1889. ... Avogadros number, also called Avogadros constant (NA) is a large constant used in chemistry and physics. ... To help compare different orders of magnitude we list here masses between 0. ...


The symbol amu for atomic mass unit can sometimes still be found, particularly in older works. Atomic masses are often written without any unit and then the atomic mass unit is implied. In biochemistry and molecular biology literature (particularly in reference to proteins), the term "dalton" is used, with the symbol "Da". Because proteins are large molecules, they are typically referred to in kilodaltons, or "kDa", with one kilodalton being equal to 1000 daltons. Biochemistry is the chemistry of life. ... Molecular biology is the study of biology at a molecular level. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... See Dalton for other senses of the word. ...


The unified atomic mass unit is not an SI unit of mass, although it is (only by that name, and only with the symbol u) accepted for use with SI. See SI website link below. The International System of Units (abbreviated SI from the French phrase, Système International dUnités) is the most widely used system of units. ...


The unit is convenient because one hydrogen atom has a mass of approximately 1 u, and more generally an atom or molecule that contains n protons and neutrons will have a mass approximately equal to n u. (The reason is that a carbon-12 atom contains 6 protons, 6 neutrons and 6 electrons, with the protons and neutrons having about the same mass and the electron mass being negligible in comparison.) This is only a rough approximation however, since it does not account for the mass contained in the binding energy of an atom's nucleus; this binding energy mass is not a fixed fraction of an atom's total mass. A hydrogen atom is an atom of the element hydrogen. ... Properties For alternative meanings see atom (disambiguation). ... A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ... Properties In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... Properties In physics, the neutron is a subatomic particle with no net electric charge and a mass of 939. ... Binding energy is the energy required to disassemble a whole into separate parts. ... A stylized representation of a lithium atom. ...


Another reason the unit is used is that it is experimentally much easier and more precise to compare masses of atoms and molecules (determine relative masses) than to measure their absolute masses. Masses are compared with a mass spectrometer (see below). Mass spectrometry is a technique for separating ions by their mass-to-charge (m/z) ratios. ...


Avogadro's number (NA) and the mole are defined so that one mole of a substance with atomic or molecular mass 1 u will have a mass of precisely 1 gram. For example, the molecular mass of water is 18.01508 u, and this means that one mole of water has a mass of 18.01508 grams, or conversely that 1 gram of water contains NA/18.01508 ≈ 3.3428 × 1022 molecules. The mole (symbol: mol) is one of the seven SI base units and is commonly used in chemistry. ... The gram or gramme, symbol g, is a unit of mass, and is defined in the SI system of units as one one-thousandth of a kilogram (i. ... Water (from the Old English word wæter; c. ...


Measuring relative atomic masses

The relative atomic mass is measured with a mass spectrometer. After placing a sample of the element to be measured in the mass spectrometer it is bombarded with electrons which turns the atoms into positive ions. An electric field is then used to accelerate these positive ions, after which the ions are deflected using a magnetic field. As a result the various isotopes are separated out due to the ions of lighter isotopes being deflected more than those heavier. This produces a mass spectrum. Mass spectrometry is a technique for separating ions by their mass-to-charge (m/z) ratios. ... The term element can refer to: Chemical element — material that consists of atoms with the same number of protons in the nucleus (see also Periodic table). ... Properties For alternative meanings see atom (disambiguation). ... An ion is an atom or group of atoms with a net electric charge. ... In physics, an electric field or E-field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity. ... Current flowing through a wire produces a magnetic field (M) around the wire. ... Isotopes are atoms of a chemical element whose nuclei have the same atomic number, Z, but different atomic weights, A. The word isotope, meaning at the same place, comes from the fact that isotopes are located at the same place on the periodic table. ... The Mass spectrum of any substance is the distribution of matter according to atomic and/or molecular masses that can be found when a sample of the substance is tested. ...


This spectrum provides two things:

  1. Relative isotopic masses in the sample
  2. Abundances of the isotopes

History

The chemist John Dalton was the first to suggest the mass of one atom of hydrogen as the atomic mass unit. Francis Aston, inventor of the mass spectrometer, later used 1/16 of the mass of one atom of oxygen-16 as his unit. A chemist is a scientist who specializes in chemistry. ... John Dalton (September 6, 1766 – July 27, 1844) was a British chemist and physicist, born at Eaglesfield, near Cockermouth in Cumberland. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... Francis William Aston (born Birmingham, September 1, 1877; died Cambridge, November 20, 1945) was a British physicist who won the 1922 Nobel Prize in Chemistry for the invention of the mass spectrometer. ... General Name, Symbol, Number oxygen, O, 8 Chemical series nonmetals Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ...


Before 1961, the physical atomic mass unit was defined as 1/16 of the mass of one atom of oxygen-16, while the chemical atomic mass unit was defined as 1/16 of the average mass of an oxygen atom (taking the natural abundance of the different oxygen isotopes into account). Both units are slightly smaller than the unified atomic mass unit, which was adopted by the International Union of Pure and Applied Physics in 1960 and by the International Union of Pure and Applied Chemistry in 1961. 1961 was a common year starting on Sunday (link will take you to calendar). ... Isotopes are forms of an element whose nuclei have the same atomic number–-the number of protons in the nucleus--but different atomic masses because the contain different numbers of neutrons. ... The International Union of Pure and Applied Physics (IUPAP) is an international non-governmental organization devoted to the advancement of Physics. ... IUPAC logo The International Union of Pure and Applied Chemistry (IUPAC) is an international non-governmental organization devoted to the advancement of chemistry. ...


External links

  • SI website on acceptable non-SI units
  • Accepted value of 1 u as of 2002

  Results from FactBites:
 
John Dalton - Wikipedia, the free encyclopedia (2534 words)
Dalton communicated his atomic theory to Thomson who, by consent, included an outline of it in the third edition of his System of Chemistry (1807), and Dalton gave a further account of it in the first part of the first volume of his New System of Chemical Philosophy (1808).
Dalton died in Manchester in 1844 of paralysis.
Dalton had requested that his eyes be examined after his death, in an attempt to discover the cause of his colour-blindness; he had hypothesised that his aqueous humour might be coloured blue.
  More results at FactBites »

 
 

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